The present invention relates to a video signal processing apparatus, a method of processing a video signal, a program for processing a video signal, and a recording medium having the program recorded therein and is applicable to a case where a motion picture is displayed in, for example, a liquid crystal display (LCD) panel. The present invention is directed to reduce motion blur by setting the pixel values of subframes such that the maximum distribution of the pixel values in a time axis direction is yielded, when one frame is displayed by using multiple subframes to represent halftones by frame rate control (FRC).
An increasing number of display devices use so-called flat panel displays (FPDs) including LCD panels, plasma display panels (PDPs), and organic electroluminescenct (EL) display panels, instead of cathode ray tubes, in recent years. Since the display devices using the LCD panels, among the display devices using the FPDs, have a smaller number of displayable tones, compared with the display devices using the cathode ray tubes, a dither method, frame rate control (FRC), and so on, which represent pseudo halftones to compensate the number of tones that is sufficient, are proposed.
The dither method represents halftones by the use of an area integration effect of the eyes of a human being. In the dither method, the pixel value of each pixel in each unit including multiple pixels is controlled to represent a halftone for every unit.
In contrast, the FRC represents the halftones by the use of a time integration effect of the eyes of a human being. In the FRC, the tones are switched for every frame to represent the halftones.
In the FRC in the past, when the pixel value of a halftone to be represented is equal to “I0”, the occurrence rate of displayable pixel values “I1” and “I2” before and after the pixel value “I0” is set to the rate according to the pixel value “I0” of the halftone, to represent the pixel value “I0” of the halftone.
For example, as shown in FIG. 14, when each tone is represented by using six bits in a display device capable of displaying each tone by using four bits, the occurrence rate of the displayable pixel values “I1” and “I2” is set to the rate according to the pixel value “I0” of the halftone in units of continuous four frames corresponding to the difference between the numbers of bits. Specifically, in the representation of the tone of a pixel value “10.75” under this condition, the three frames, among the continuous four frames, are represented with a pixel value “11” and the remaining one frame is represented with a pixel value “10”.
In the following examples including the example in FIG. 14, the brightness of each pixel is represented by a pixel value with respect to the brightest pixel value, among the pixel values represented by the tone values. For example, as shown in FIG. 14, the brightest pixel value is represented by “15” in the display of 16 tones by using four bits. As shown in FIG. 15, the brightest pixel value is represented by “255” in the display of 256 tones by using eight bits, and the brightest pixel value is represented by “63” in the display of 64 tones by using six bits. These brightest pixel values may be displayed along with the pixel values indicating the brightness of the pixels, if required.
The display of the halftones by the FRC has a disadvantage of a flicker that is highly visible. In order to resolve such a problem, for example, the Japanese Examined Patent Application Publication No. 7-89265 discloses a technique for making the flicker indistinctive by using the FRC with the dither method.
In recent years, display panels having higher response speeds in, for example, an optical compensated birefringence (OCB) mode have been developed. Methods of displaying one frame by using multiple subframes in a display panel having a higher response speed to represent the halftones by the FRC are also proposed. According to Bloch's law, since it is difficult for the eyes of a human being to recognize a variation in light incident over a predetermined time period, the eyes of the human being recognize only the integrated value of light incident over the predetermined time period. Accordingly, increasing the frame frequency and representing the halftones by the FRC allow the flicker to be made indistinctive.
Specifically, in order to display a video signal S1 with 256 tones in a display panel that can display only up to 64 tones, one frame of the video signal S1 is displayed by using four subframes, as shown by arrows in FIG. 15, to display the video image corresponding to the video signal S1 by using a video signal S2 having the frame frequency four times higher than that of the video signal S1. In addition, the pixel value of each pixel in the four subframes is set to the displayable pixel value “I1” or “I2” before or after the pixel value “I0” of the halftone corresponding to the original video signal S1, and the occurrence rate of the pixel value “I1” and “I2” in the four subframes is set to the rate according to the pixel value “I0” of the halftone corresponding to the original video signal S1. As a result, when the pixel value of the original video signal S1 is, for example, “98/255”, the pixels corresponding to the four subframes can be represented by pixel values “24”, “25”, “24”, and “25” to make the flicker indistinctive and to represent the halftone by a pixel value “24.5/63 (98/255)”, which is the average of the pixel values of the tones of the four subframes.
In a hold-type display device, such as the LCD panel, the same image is continued to be displayed during one frame. Accordingly, when a human being follows an object that is moving with his eye, the position where an image of the object is formed (hereinafter referred to as an image forming position) vibrates on the retina. As a result, the image of the moving object is blurred to cause so-called motion blur. This vibration is caused by repetition of an operation in which, after the image forming position is shifted in a direction opposite to the moving direction of the object during one frame, the position instantaneously returns to the original image forming position.
Such motion blur does not occur in impulse-type display devices, such as the cathode ray tube. Accordingly, techniques for approximating the display characteristics of the LCD devices to those of the impulse-type display devices by driving the LCD panel or by backlight control are proposed in order to reduce the motion blur.
The techniques adopting the drive of the LCD panel is called black insertion in which fully black subframes are inserted between frames. Although these techniques can prevent the motion blur, there is a problem of reduction in the brightness. In contrast, the techniques adopting the backlight control achieve an effect similar to that of the black insertion by intermittently turning on the backlight.
There are cases in which motion pictures are displayed in the display devices described above. FIG. 16 shows a display image of an object 1 that is moving from left to right, as shown by an arrow. The motion of an edge of the moving object 1 is represented by continuous frames, as shown by reference letters and numerals F1 and F2, which denote enlarged continuous frames.
As shown in FIG. 17 in contrast to FIG. 15, when one frame is displayed by using the multiple subframes to represent the halftones by the FRC, the motion of the edge of the moving object 1 is intermittently represented by using the four subframes. When a human being follows the moving object 1 with his eye, as shown in FIG. 18A, the position where an image of the moving object 1 is formed vibrates on the retina every four frames, as shown in FIG. 18B. As a result, the motion blur is also disadvantageously caused.
The vibration of the image forming position of the moving object 1 on the retina due to the motion blur is caused by repetition of an operation in which, after the image forming position of the moving object 1 is shifted stepwise in a direction opposite to the moving direction of the moving object 1 by a distance corresponding to the multiple subframes allocated to one frame, the position instantaneously returns to the original image forming position. Referring to FIG. 18B, when the subframes are displayed under the condition described above with reference to FIG. 17, the pixel values on the retina are represented by the tones of the original video signal S1.
In order to resolve the above problems, a method of generating these subframes by frame interpolation using motion vectors is disclosed in, for example, Japanese Patent No. 3158904. However, the frame interpolation using the motion vectors causes a problem in that the structure of the display device becomes complicated. Furthermore, it may be impossible to completely prevent the motion vectors from being incorrectly detected. If the motion vectors are incorrectly detected, the motion blur is increased to display a significantly unnatural image.